1. Field of the Invention
The present invention pertains to a device having a selectively adjustable eccentric with a self-locking feature operatively connected to a connecting rod that initiates activation of a sewing machine's feed dog, thereby, controlling the rate of advance of a workpiece through the sewing zone and the length of stitch formed in the workpiece.
2. Description of the Prior Art
Stitch adjusting devices are well known to those conversant in the art which utilize an adjustable eccentric carried by a support collar that is fixed on a sewing machine's main shaft. Such devices include a control eccentric in operative association with the adjustable eccentric which has a connecting rod for advancing the feed dog that can be selectively positioned eccentrically on the support collar with respect to the main shaft. The control eccentric has an elongated inner passage which houses the adjustable eccentric that forms an integral part of a support ring which is rotatably mounted on the machine's main shaft. This support ring is adapted to be held stationary by means of a push button operable externally of the machine or which is connected to a member that is fixed to the machine's main shaft whereby the means connecting the ring to the member define toothed elements. These toothed elements have unidirectional teeth or, in other words, sawtooth shaped teeth which are connected to the support ring and are urged by spring means against teeth of conforming configuration on the member fixed to the machine's main shaft.
A device of the above type is shown and described in U.S. Pat. No. 2,161,579 which satisfactorily performs its intended function but presents a problem when the feed dog is subjected to heavy material or during the crossing of seams by becoming blocked with the result that a condition develops whereby the toothed element slides on the teeth of conforming configuration on the member fixed to the machine's main shaft which overrides the pressure of the spring. A condition of this nature inevitably leads to the lengthening of the stitch since the eccentric, not being firmly positioned, has a tendency to become displaced externally with respect to the shaft.
Another device of the above type is shown and described in U.S. Pat. No. 2,491,449 which includes a member with forwardly disposed teeth fixed on the main shaft of the machine. Within this member an element is engaged which is pivoted on a ring that is free to rotate with respect to the main shaft. This device also performs its intended function satisfactorily but presents a problem when attempting to obtain a fine setting of the adjustable eccentric. Such a condition can be attributed to the nature of the teeth which cannot be provided with a fine pitch.
An object of the invention is to provide a stitch adjustment device which, upon obtaining a selected eccentricity that corresponds to a desired stitch length, will be maintained unaltered under all possible sewing conditions including the stitching of heavy material and the crossing of seams.
Another object is to provide a stitch adjustment device which will provide every possible adjustment for a desired stitch length and with the ability of obtaining infinitesimally small values thereof.